In general, a movement path of a tool, a speed of the tool, and the like in a machine tool are numerically controlled, so that a cutting process is performed on a workpiece. Examples of the machine tool include a machining center, a turning center, and an NC milling machine. The machine tool is also referred to as a numerical control complex processing machine.
Among the tools, there is a rotation cutting tool which rotates in a state of being mounted in a spindle and performs a cutting process on a stopped workpiece. Examples of the rotation cutting tool include an end mill, a face mill, a milling tool, a drill tool, and a boring tool. Hereinafter, the “rotation cutting tool” is abbreviated and called the “tool”.
The machine tool is evaluated to have excellent productivity when a material removal rate (MRR) is high, and when surface roughness of a processed surface is low, the machine tool is evaluated to have an excellent processing quality.
The MRR is determined according to a cutting condition element, such as a cutting depth in a radius direction, a cutting depth in an axis direction, a speed of a main shaft, and a feed rate of a main shaft. Here, the speed of the main shaft and the speed of the spindle have the same meaning. Further, when the spindle is rotated in a state where the tool is mounted on the spindle, the tool is rotated, so that it may be understood that the speed of the tool and the speed of the main shaft have the same numerical value. Further, it may be understood that a feed rate of the main shaft and a feed rate of the tool have the same value. The MRR may be expressed by Equation 1 below.MRR=R×A×V  [Equation 1]
MRR: Material removal rate (mm3/min)
R: Cutting depth in radius direction (mm)
A: Cutting depth in axis direction (mm)
V: Feed rate of main shaft (mm/min)
That is, according to Equation 1, even any one among the cutting condition elements is increased, the MRR is increased. In the meantime, there is a case where a cutting condition of a tool is represented by a tool manufacturing company as a manual recommendation condition. However, the manual recommendation condition is provided based on a maximally allowed static processing load, and is a value, to which a chatter vibration characteristic generated during the cutting process is not reflected.
Accordingly, there is a problem in that it is impossible to handle chatter vibrations generated during the cutting process and damage of a tool or a workpiece according to a dynamic processing load amplified by the chatter vibration. Because of this, an operator tends to perform the cutting process by setting a cutting condition to a very stable cutting condition instead of the manual recommended condition.
Further, the process condition includes the type of tool, a tool shape, a protruding length of a tool, the number of blades of a tool, hardness of a workpiece, a position (a position of X, Y, and Z coordinates) of a processing point within a machine tool, and the chatter vibration has a characteristic continuously changed according to the process condition.
That is, in order to secure a process quality, an operator tends to apply a very conservative cutting condition among the manual recommended conditions represented by a tool manufacturing company, thereby causing a problem of degrading productivity.
In order to improve both productivity and a process quality, it is demanded to suppress and avoid vibrations by continuously evaluating and analyzing a vibration characteristic during a progress of a cutting process.